The present invention relates to a process for simultaneously preparing organosiloxanes and alkyl halides from the corresponding dialkyldialkoxysilane, and more particularly, to a process for controlling the organosiloxane and alkyl halide products from the reaction of dialkyldialkoxysilanes and hydrogen halides.
Generally, organopolysiloxanes are made by the hydrolysis of one or a mixture of organochlorosilanes to form silanes having silanol groups, which, in turn, form silanol condensation products to produce the organopolysiloxanes. The organopolysiloxanes are useful as lubricants and for the preparation of high molecular weight silicone products by conventional and well-known methods.
One of the by-products formed by the hydrolysis of the organochlorosilane is hydrogen chloride. In commercial practice, aqueous hydrogen chloride is distilled to recover the anhydrous form which is reacted with methyl alcohol to produce water and methyl chloride. The methyl chloride is further used to produce alkylchlorosilanes by reacting the methyl chloride with metallic silicon. In the foregoing method, the organopolysiloxanes and the methyl chloride are generally produced in two separate processes.
The concurrent preparation of methyl chloride and organosiloxanes has been proposed and is disclosed in U.S. Pat. No. 4,366,324. In U.S. Pat. No. 4,366,324, there is a direct reaction between an organohalogenosiloxane and an alkanol to produce an alkyl halide and organopolysiloxanes, or, more particularly, there is a direct reaction between an organochlorosilane and methyl alcohol to produce organopolysiloxanes and methyl chloride. By this process, large volumes of hydrochloric acid are avoided, however, generally, the organochlorosilanes used for hydrolysis contain methyltricholosilane which is responsible for the formation of gels. Thus, it is desirable to provide a process wherein the starting materials are free of tri-functional impurities, such as methyltrichlorosilane, to avoid the formation of tri-functional sites upon monomers which are responsible for the formation of the gels.
Various processes have been proposed for making organopolysiloxanes from dialkyldialkoxysilanes. In U.S. Pat. No. 2,465,547, the cyclic octomer, hexadecamethylcyclooctasiloxane, is produced in the acid catalyzed hydrolysis of dimethyldiethoxysilane or dimethyldichlorosilane. The dimethyldiethoxysilane is mixed with one volume of a mixture of 95% ethyl alcohol and concentrated aqueous hydrochloric acid in equal parts to prepare the cyclic polymer after the reaction mixture is refluxed for about 4 to 8 hours. The polymer product is a mixture of cyclic polymers of dimethylsiloxane having up to 13 silicon atoms per molecule. The cyclic octamer is obtained at certain temperatures and pressures by distillation.
In a similar process, as described in U.S. Pat. No. 2,384,384, dimethyl diethoxy silicane dissolved in ethyl alcohol, is hydrolyzed with water, and preferably, the hydrolysis is carried out in the presence of an acid catalyst by mixing one volume of the dimethyldiethoxysilicane with one volume of a mixture of 95% ethyl alcohol and concentrated aqueous hydrochloric acid in equal parts to prepare mixtures of dimethylsilicone polymers, the lower polymers containing up to about 8 of the siloxane structural units wherein the siloxane units are joined in a ring of siloxane linkages. The hydrolysis by this method is disadvantageous for the continuous production of silicones because the reaction mixture must be refluxed for at least 1 hour and, more preferably, about 4 to 8 hours, to obtain the products. Furthermore, the process of U.S. Pat. No. 2,384,384 is disadvantageous because ethyl alcohol must be added to the reaction mixture, and little or no alkyl halide is formed as a result of the reaction because only catalytic amounts of the aqueous hydrochloric acid are used.
Reactions between tri-functional silanes, such as trimethoxysilane, and hydrogen chloride are disclosed in U.S. Pat. No. 3,567,756. However, the products of the reaction are monochlorotrimethoxysilane and hydrogen. Furthermore, as discussed above, dialkyldialkoxysilanes having trifunctional impurities promote the formation of gels upon hydrolysis. Accordingly, any trifunctional alkoxysilane impurity in the dialkyldialkoxysilane monomer is disadvantageous in the production of organopolysiloxanes.
Alternative hydrolysis methods and methods of making polysiloxanes from dialkyldialkoxysilanes are disclosed in U.S. Pat. No. 2,439,856, U.S. Pat. No. 2,719,859 and U.S. Pat. No. 2,731,485. The processes disclosed in these patents generally have most of the same disadvantages discussed above. Other processes and the disadvantages thereof, are disclosed and discussed in U.S. Pat. No. 4,366,324 briefly discussed above.